Light microscopic autoradiographic localization of [3H]oxytocin binding sites in the rat brain, pituitary and mammary gland

Behavioral plasticity: Role of neuropeptides in shaping feeding responses

Behavioral plasticity refers to changes occurring due to external influences on an organism, including adaptation, learning, memory and enduring influences from early life experience. There are 2 types of behavioral plasticity: "developmental", which refers to gene/environment interactions affecting a phenotype, and "activational" which refers to innate physiology and can involve structural physiological changes of the body. In this review, we focus on feeding behavior, and studies involving neuropeptides that influence behavioral plasticity - primarily opioids, orexin, neuropeptide Y, and oxytocin. In each section of the review, we include examples of behavioral plasticity as it relates to actions of these neuropeptides. It can be concluded from this review that eating behavior is influenced by a number of external factors, including time of day, type of food available, energy balance state, and stressors. The reviewed work underscores that environmental factors play a critical role in feeding behavior and energy balance, but changes in eating behavior also result from a multitude of non-environmental factors, such that there can be no single mechanism or variable that can explain ingestive behavior.

Using Receptor Autoradiography to Visualize and Quantify Oxytocin and Vasopressin 1a Receptors in the Human and Nonhuman Primate Brain

Despite its development almost 40 years ago, receptor autoradiography remains a regular and reliable practice for the localization of oxytocin and vasopressin receptors in brain tissue sections. It is used across many laboratories, institutions, and animal species to characterize and quantify the distribution and density of these receptors at baseline and/or in response to experimental manipulations or lived experience. This powerful tool and the neuroanatomical receptor maps that it generates have allowed researchers to more accurately investigate and understand the neural substrates upon which oxytocin and vasopressin act to affect behavior. Researchers have used these maps to design site-specific pharmacological manipulations and electrophysiological recordings in animal studies to directly probe the underlying neural mechanisms in this system. This methods chapter describes the specific procedures by which a pharmacologically optimized, competitive binding modification to receptor autoradiography can be used to reliably localize oxytocin and vasopressin receptors in the human brain and in the brains of nonhuman primates. The ability to reliably perform receptor autoradiography for these targets in human brain tissue can finally inform our interpretation of past intranasal oxytocin neuroimaging studies and allows us to move past the reliance on transcriptomic studies using brain tissue homogenates so that we can directly investigate the involvement of oxytocin and vasopressin receptors in human behavior, physiology, and neuropsychiatric disease.

Oxytocin as an Anti-obesity Treatment

Obesity is a growing health concern, as it increases risk for heart disease, hypertension, type 2 diabetes, cancer, COVID-19 related hospitalizations and mortality. However, current weight loss therapies are often associated with psychiatric or cardiovascular side effects or poor tolerability that limit their long-term use. The hypothalamic neuropeptide, oxytocin (OT), mediates a wide range of physiologic actions, which include reproductive behavior, formation of prosocial behaviors and control of body weight. We and others have shown that OT circumvents leptin resistance and elicits weight loss in diet-induced obese rodents and non-human primates by reducing both food intake and increasing energy expenditure (EE). Chronic intranasal OT also elicits promising effects on weight loss in obese humans. This review evaluates the potential use of OT as a therapeutic strategy to treat obesity in rodents, non-human primates, and humans, and identifies potential mechanisms that mediate this effect.

Early experience alters developmental trajectory of central oxytocin systems involved in hypothalamic-pituitary-adrenal axis regulation in Long-Evans rats

Oxytocin is important for postnatal developmental experiences for mothers, infants, and transactions between them. Oxytocin is also implicated in adult affiliative behaviors, including social buffering of stress. There is evidence for connections between early life experience and adult oxytocin system functioning, but effects of early experience on behavioral, endocrine, and neurophysiological outcomes related to adult social buffering are not well explored. We use a limited bedding and nesting (LBN) material paradigm as an environmental disruption of early experiences and assessed central oxytocin systems in brain regions related to hypothalamic-pituitary-adrenal (HPA) axis regulation (paraventricular nucleus of the hypothalamus, amygdala, hippocampus). We also assessed developmentally-appropriate social behaviors and HPA reactivity during social buffering testing in adulthood. LBN litters had larger huddles and more pups visible compared to control litters during the first two weeks of life. LBN also altered the developmental trajectory of oxytocin-expressing cells and oxytocin receptor cells, with increases in oxytocin receptor cells at P15 in LBN pups. By adulthood, LBN females had more and LBN males had fewer oxytocin and oxytocin receptor cells in these areas compared to sex-matched controls. Adult LBN females, but not LBN males, had behavioral changes during social interaction and social buffering testing. The sex-specific effects of early experience on central oxytocin systems and social behavior may contribute to female resilience to early life adversity.

The role of oxytocin in regulation of appetitive behaviour, body weight and glucose homeostasis

Obesity and its associated complications have reached epidemic proportions in the USA and also worldwide, highlighting the need for new and more effective treatments. Although the neuropeptide oxytocin (OXT) is well recognised for its peripheral effects on reproductive behaviour, the release of OXT from somatodendrites and axonal terminals within the central nervous system (CNS) is also implicated in the control of energy balance. In this review, we summarise historical data highlighting the effects of exogenous OXT as a short-term regulator of food intake in a context-specific manner and the receptor populations that may mediate these effects. We also describe what is known about the physiological role of endogenous OXT in the control of energy balance and whether serum and brain levels of OXT relate to obesity on a consistent basis across animal models and humans with obesity. We describe recent data on the effectiveness of chronic CNS administration of OXT to decrease food intake and weight gain or to elicit weight loss in diet-induced obese (DIO) and genetically obese mice and rats. Of clinical importance is the finding that chronic central and peripheral OXT treatments both evoke weight loss in obese animal models with impaired leptin signalling at doses that are not associated with visceral illness, tachyphylaxis or adverse cardiovascular effects. Moreover, these results have been largely recapitulated following chronic s.c. or intranasal treatment in DIO non-human primates (rhesus monkeys) and obese humans, respectively. We also identify plausible mechanisms that contribute to the effects of OXT on body weight and glucose homeostasis in rodents, non-human primates and humans. We conclude by describing the ongoing challenges that remain before OXT-based therapeutics can be used as a long-term strategy to treat obesity in humans.

Characterization of Oxytocin Receptor Expression Within Various Neuronal Populations of the Mouse Dorsal Hippocampus

Oxytocin, acting through the oxytocin receptor (Oxtr) in the periphery, is best known for its roles in regulating parturition and lactation. However, it is also now known to possess a number of important social functions within the central nervous system, including social preference, memory and aggression, that vary to different degrees in different species. The Oxtr is found in both excitatory and inhibitory neurons within the brain and research is focusing on how, for example, activation of the receptor in interneurons can enhance the signal-to-noise of neuronal transmission. It is important to understand which neurons in the mouse dorsal hippocampus might be activated during memory formation. Therefore, we examined the colocalization of transcripts in over 5,000 neurons for Oxtr with those for nine different markers often found in interneurons using hairpin chain reaction in situ hybridization on hippocampal sections. Most pyramidal cell neurons of CA2 and many in the CA3 express Oxtr. Outside of those excitatory neurons, over 90% of Oxtr-expressing neurons co-express glutamic acid decarboxylase-1 (Gad-1) with progressively decreasing numbers of co-expressing cholecystokinin, somatostatin, parvalbumin, neuronal nitric oxide synthase, the serotonin 3a receptor, the vesicular glutamate transporter 3, calbindin 2 (calretinin), and vasoactive intestinal polypeptide neurons. Distributions were analyzed within hippocampal layers and regions as well. These findings indicate that Oxtr activation will modulate the activity of ~30% of the Gad-1 interneurons and the majority of the diverse population of those, mostly, interneuron types specifically examined in the mouse hippocampus.

The effects of oxytocin on eating behaviour and metabolism in humans

Oxytocin, a hypothalamic hormone that is secreted directly into the brain and enters the peripheral circulation through the posterior pituitary gland, regulates a range of physiologic processes, including eating behaviour and metabolism. In rodents and nonhuman primates, chronic oxytocin administration leads to sustained weight reduction by reducing food intake, increasing energy expenditure and inducing lipolysis. Oxytocin might improve glucose homeostasis, independently of its effects on weight. Clinical studies are beginning to translate these important preclinical findings to humans. This Review describes key data linking oxytocin to eating behaviour and metabolism in humans. For example, a single intranasal dose of oxytocin can reduce caloric intake, increase fat oxidation and improve insulin sensitivity in men. Furthermore, a pilot study of 8 weeks of oxytocin treatment in adults with obesity or overweight led to substantial weight loss. Together, these data support further investigation of interventions that target pathways involving oxytocin as potential therapeutics in metabolic disorders, including obesity and diabetes mellitus. Therapeutic considerations and areas for further research are also discussed.

Oxytocin Acting in the Nucleus Accumbens Core Decreases Food Intake

Central oxytocin (OT) promotes feeding termination in response to homeostatic challenges, such as excessive stomach distension, salt loading and toxicity. OT has also been proposed to affect feeding reward by decreasing the consumption of palatable carbohydrates and sweet tastants. Because the OT receptor (OTR) is expressed in the nucleus accumbens core (AcbC) and shell (AcbSh), a site regulating diverse aspects of eating behaviour, we investigated whether OT acts there to affect appetite in rats. First, we examined whether direct AcbC and AcbSh OT injections affect hunger- and palatability-driven consumption. We found that only AcbC OT infusions decrease deprivation-induced chow intake and reduce the consumption of palatable sucrose and saccharin solutions in nondeprived animals. These effects were abolished by pretreatment with an OTR antagonist, L-368,899, injected in the same site. AcbC OT at an anorexigenic dose did not induce a conditioned taste aversion, which indicates that AcbC OT-driven anorexia is not caused by sickness/malaise. The appetite-specific effect of AcbC OT is supported by the real-time polymerase chain reaction analysis of OTR mRNA in the AcbC, which revealed that food deprivation elevates OTR mRNA expression, whereas saccharin solution intake decreases OTR transcript levels. We also used c-Fos immunohistochemistry as a marker of neuronal activation and found that AcbC OT injection increases activation of the AcbC itself, as well as of two feeding-related sites: the hypothalamic paraventricular and supraoptic nuclei. Finally, considering the fact that OT plays a significant role in social behaviour, we examined whether offering animals a meal in a social setting would modify their hypophagic response to AcbC OT injections. We found that a social context abolishes the anorexigenic effects of AcbC OT. We conclude that OT acting via the AcbC decreases food intake driven by hunger and reward in rats offered a meal in a nonsocial setting.

Distinct BOLD Activation Profiles Following Central and Peripheral Oxytocin Administration in Awake Rats

A growing body of literature has suggested that intranasal oxytocin (OT) or other systemic routes of administration can alter prosocial behavior, presumably by directly activating OT sensitive neural circuits in the brain. Yet there is no clear evidence that OT given peripherally can cross the blood-brain barrier at levels sufficient to engage the OT receptor. To address this issue we examined changes in blood oxygen level-dependent (BOLD) signal intensity in response to peripheral OT injections (0.1, 0.5, or 2.5 mg/kg) during functional magnetic resonance imaging (fMRI) in awake rats imaged at 7.0 T. These data were compared to OT (1 μg/5 μl) given directly to the brain via the lateral cerebroventricle. Using a 3D annotated MRI atlas of the rat brain segmented into 171 brain areas and computational analysis, we reconstructed the distributed integrated neural circuits identified with BOLD fMRI following central and peripheral OT. Both routes of administration caused significant changes in BOLD signal within the first 10 min of administration. As expected, central OT activated a majority of brain areas known to express a high density of OT receptors, e.g., lateral septum, subiculum, shell of the accumbens, bed nucleus of the stria terminalis. This profile of activation was not matched by peripheral OT. The change in BOLD signal to peripheral OT did not show any discernible dose-response. Interestingly, peripheral OT affected all subdivisions of the olfactory bulb, in addition to the cerebellum and several brainstem areas relevant to the autonomic nervous system, including the solitary tract nucleus. The results from this imaging study do not support a direct central action of peripheral OT on the brain. Instead, the patterns of brain activity suggest that peripheral OT may interact at the level of the olfactory bulb and through sensory afferents from the autonomic nervous system to influence brain activity.

Ontogenesis of oxytocin pathways in the mammalian brain: late maturation and psychosocial disorders

Oxytocin (OT), the main neuropeptide of sociality, is expressed in neurons exclusively localized in the hypothalamus. During the last decade, a plethora of neuroendocrine, metabolic, autonomic and behavioral effects of OT has been reported. In the urgency to find treatments to syndromes as invalidating as autism, many clinical trials have been launched in which OT is administered to patients, including adolescents and children. However, the impact of OT on the developing brain and in particular on the embryonic and early postnatal maturation of OT neurons, has been only poorly investigated. In the present review we summarize available (although limited) literature on general features of ontogenetic transformation of the OT system, including determination, migration and differentiation of OT neurons. Next, we discuss trajectories of OT receptors (OTR) in the perinatal period. Furthermore, we provide evidence that early alterations, from birth, in the central OT system lead to severe neurodevelopmental diseases such as feeding deficit in infancy and severe defects in social behavior in adulthood, as described in Prader-Willi syndrome (PWS). Our review intends to propose a hypothesis about developmental dynamics of central OT pathways, which are essential for survival right after birth and for the acquisition of social skills later on. A better understanding of the embryonic and early postnatal maturation of the OT system may lead to better OT-based treatments in PWS or autism.

Overexpression of oxytocin receptors in the hypothalamic PVN increases baroreceptor reflex sensitivity and buffers BP variability in conscious rats

BACKGROUND AND PURPOSE

The paraventricular nucleus (PVN) of the hypothalamus is an important integrative site for neuroendocrine control of the circulation. We investigated the role of oxytocin receptors (OT receptors) in PVN in cardiovascular homeostasis.

EXPERIMENTAL APPROACH

Experiments were performed in conscious male Wistar rats equipped with a radiotelemetric device. The PVN was unilaterally co-transfected with an adenoviral vector (Ad), engineered to overexpress OT receptors, and an enhanced green fluorescent protein (eGFP) tag. Control groups: PVN was transfected with an Ad expressing eGFP alone or untransfected, sham rats (Wt). Recordings were obtained without and with selective blockade of OT receptors (OTX), during both baseline and stressful conditions. Baroreceptor reflex sensitivity (BRS) and cardiovascular short-term variability were evaluated using the sequence method and spectral methodology respectively.

KEY RESULTS

Under baseline conditions, rats overexpressing OT receptors (OTR) exhibited enhanced BRS and reduced BP variability compared to control groups. Exposure to stress increased BP, BP variability and HR in all rats. In control groups, but not in OTR rats, BRS decreased during stress. Pretreatment of OTR rats with OTX reduced BRS and enhanced BP and HR variability under baseline and stressful conditions. Pretreatment of Wt rats with OTX, reduced BRS and increased BP variability under baseline and stressful conditions, but only increased HR variability during stress.

CONCLUSIONS AND IMPLICATIONS

OT receptors in PVN are involved in tonic neural control of BRS and cardiovascular short-term variability. The failure of this mechanism could critically contribute to the loss of autonomic control in cardiovascular disease.

Oxytocin and vasopressin modulation of the neural correlates of motivation and emotion: results from functional MRI studies in awake rats

Oxytocin and vasopressin modulate a range of species typical behavioral functions that include social recognition, maternal-infant attachment, and modulation of memory, offensive aggression, defensive fear reactions, and reward seeking. We have employed novel functional magnetic resonance mapping techniques in awake rats to explore the roles of these neuropeptides in the maternal and non-maternal brain. Results from the functional neuroimaging studies that are summarized here have directly and indirectly confirmed and supported previous findings. Oxytocin is released within the lactating rat brain during suckling stimulation and activates specific subcortical networks in the maternal brain. Both vasopressin and oxytocin modulate brain regions involved unconditioned fear, processing of social stimuli and the expression of agonistic behaviors. Across studies there are relatively consistent brain networks associated with internal motivational drives and emotional states that are modulated by oxytocin and vasopressin. This article is part of a Special Issue entitled Oxytocin and Social Behav.

Oxytocin, motivation and the role of dopamine

The hypothalamic neuropeptide oxytocin has drawn the attention of scientists for more than a century. The understanding of the function of oxytocin has expanded dramatically over the years from a simple peptide adept at inducing uterine contractions and milk ejection to a complex neuromodulator with a capacity to shape human social behavior. Decades of research have outlined oxytocin's ability to enhance intricate social activities ranging from pair bonding, sexual activity, affiliative preferences, and parental behaviors. The precise neural mechanisms underlying oxytocin's influence on such behaviors have just begun to be understood. Research suggests that oxytocin interacts closely with the neural pathways responsible for processing motivationally relevant stimuli. In particular, oxytocin appears to impact dopaminergic activity within the mesocorticolimbic dopamine system, which is crucial not only for reward and motivated behavior but also for the expression of affiliative behaviors. Though most of the work performed in this area has been done using animal models, several neuroimaging studies suggest similar relationships may be observed in humans. In order to introduce this topic further, this paper will review the recent evidence that oxytocin may exert some of its social-behavioral effects through its impact on motivational networks.

Neuroendocrine aspects of Tourette syndrome

There is sparse evidence suggesting the participation of neuroendocrine mechanisms, mainly involving sex and stress steroid hormones, to the pathophysiology of neurodevelopmental disorders such as Tourette syndrome (TS) and obsessive-compulsive disorder (OCD). Patients with TS exhibit a sex-specific variability in gender distribution (male/female ratio=3-4/1) and in its natural history, with a severity peak in the period around puberty. The administration of exogenous androgens may worsen tics in males with TS, whereas drugs counteracting the action of testosterone might show some antitic efficacy. This suggests a higher susceptibility of patients with TS to androgen steroids. There are insufficient data on the regulation of the hypothalamic-pituitary-gonadal (HPG) axis in TS. However, preliminary evidence suggests that a subgroup of women with TS might be more sensitive to the premenstrual trough of estrogen levels. Patients with TS exhibit differences in a number of behavioral, cognitive, and anatomical traits that appear to be sex related. There is a body of evidence supporting, albeit indirectly, the hypothesis of an increased exposure to androgenic steroids during the very early phases of neural development. Animal models in rodents suggest a complex role of gonadal hormones upon the modulation of anxiety-related and stereotyped behaviors during adult life. Patients with TS exhibit an enhanced reactivity of the hypothalamic-pituitary-adrenal axis to external stressors, despite a preserved diurnal cortisol rhythm and a normal restoration of the baseline activity of the axis following the acute stress response. Preliminary evidence suggests the possible implication of oxytocin (OT) in disorders related to the TS spectrum, especially non-tic-related OCD. The injection of OT in the amygdala of rodents was shown to be able to induce hypergrooming, suggesting the possible involvement of this neuropeptide in the pathophysiology of complex, stereotyped behaviors. In contrast, there is anecdotal clinical evidence that tics improve following periods of affectionate touch and sexual intercourse.

Oxytocin gene polymorphisms influence human dopaminergic function in a sex-dependent manner

BACKGROUND

Oxytocin, classically involved in social and reproductive activities, is increasingly recognized as an antinociceptive and anxiolytic agent, effects which may be mediated via oxytocin's interactions with the dopamine system. Thus, genetic variation within the oxytocin gene (OXT) is likely to explain variability in dopamine-related stress responses. As such, we examined how OXT variation is associated with stress-induced dopaminergic neurotransmission in a healthy human sample.

METHODS

Fifty-five young healthy volunteers were scanned using [¹¹C]raclopride positron emission tomography while they underwent a standardized physical and emotional stressor that consisted of moderate levels of experimental sustained deep muscle pain, and a baseline, control state. Four haplotype tagging single nucleotide polymorphisms located in regions near OXT were genotyped. Measures of pain, affect, anxiety, well-being and interpersonal attachment were also assessed.

RESULTS

Female rs4813625 C allele carriers demonstrated greater stress-induced dopamine release, measured as reductions in receptor availability from baseline to the pain-stress condition relative to female GG homozygotes. No significant differences were detected among males. We also observed that female rs4813625 C allele carriers exhibited higher attachment anxiety, higher trait anxiety and lower emotional well-being scores. In addition, greater stress-induced dopamine release was associated with lower emotional well-being scores in female rs4813625 C allele carriers.

CONCLUSIONS

Our results suggest that variability within the oxytocin gene appear to explain interindividual differences in dopaminergic responses to stress, which are shown to be associated with anxiety traits, including those linked to attachment style, as well as emotional well-being in women.

Adjunctive intranasal oxytocin reduces symptoms in schizophrenia patients

BACKGROUND

Both human and animal studies suggest oxytocin may have antipsychotic properties. Therefore, we conducted a clinical trial to directly test this notion.

METHODS

Nineteen schizophrenia patients with residual symptoms despite being on a stable dose of at least one antipsychotic were enrolled in a randomized, double-blind, crossover study. They received 3 weeks of daily intranasal oxytocin (titrated to 40 IU twice a day) and placebo adjunctive to their antipsychotics. Order of intranasal treatment was randomly assigned and there was a 1-week washout between treatments.

RESULTS

Analysis of the 15 subjects who completed all the study visits revealed that oxytocin significantly reduced scores on the Positive and Negative Symptom Scale (p < .001) and Clinical Global Impression-Improvement Scale (p < .001) compared with placebo at the 3-week end point. No benefit was seen at the early time points. Oxytocin was well tolerated and produced no adverse effects based upon patient reports or laboratory analysis.

CONCLUSIONS

The results support the hypothesis that oxytocin has antipsychotic properties and is well tolerated. Higher doses and longer duration of treatment may produce larger benefits and should be evaluated in future studies.

Dopamine and oxytocin interactions underlying behaviors: potential contributions to behavioral disorders

Dopamine is an important neuromodulator that exerts widespread effects on the central nervous system (CNS) function. Disruption in dopaminergic neurotransmission can have profound effects on mood and behavior and as such is known to be implicated in various neuropsychiatric behavioral disorders including autism and depression. The subsequent effects on other neurocircuitries due to dysregulated dopamine function have yet to be fully explored. Due to the marked social deficits observed in psychiatric patients, the neuropeptide, oxytocin is emerging as one particular neural substrate that may be influenced by the altered dopamine levels subserving neuropathologic-related behavioral diseases. Oxytocin has a substantial role in social attachment, affiliation and sexual behavior. More recently, it has emerged that disturbances in peripheral and central oxytocin levels have been detected in some patients with dopamine-dependent disorders. Thus, oxytocin is proposed to be a key neural substrate that interacts with central dopamine systems. In addition to psychosocial improvement, oxytocin has recently been implicated in mediating mesolimbic dopamine pathways during drug addiction and withdrawal. This bi-directional role of dopamine has also been implicated during some components of sexual behavior. This review will discuss evidence for the existence dopamine/oxytocin positive interaction in social behavioral paradigms and associated disorders such as sexual dysfunction, autism, addiction, anorexia/bulimia, and depression. Preliminary findings suggest that whilst further rigorous testing has to be conducted to establish a dopamine/oxytocin link in human disorders, animal models seem to indicate the existence of broad and integrated brain circuits where dopamine and oxytocin interactions at least in part mediate socio-affiliative behaviors. A profound disruption to these pathways is likely to underpin associated behavioral disorders. Central oxytocin pathways may serve as a potential therapeutic target to improve mood and socio-affiliative behaviors in patients with profound social deficits and/or drug addiction.

Oxytocin as feeding inhibitor: maintaining homeostasis in consummatory behavior

Initial studies showed that the anorexigenic peptide oxytocin (OT) regulates gastric motility, responds to stomach distention and to elevated osmolality, and blocks consumption of toxic foods. Most recently, it has been proposed to act as a mediator of general and carbohydrate-specific satiety and regulator of body weight. In the current review, we discuss the function of OT as a homeostatic inhibitor of consumption, capable of mitigating multiple aspects of ingestive behavior and energy metabolism.

Oxytocin: the great facilitator of life

Oxytocin (Oxt) is a nonapeptide hormone best known for its role in lactation and parturition. Since 1906 when its uterine-contracting properties were described until 50 years later when its sequence was elucidated, research has focused on its peripheral roles in reproduction. Only over the past several decades have researchers focused on what functions Oxt might have in the brain, the subject of this review. Immunohistochemical studies revealed that magnocellular neurons of the hypothalamic paraventricular and supraoptic nuclei are the neurons of origin for the Oxt released from the posterior pituitary. Smaller cells in various parts of the brain, as well as release from magnocellular dendrites, provide the Oxt responsible for modulating various behaviors at its only identified receptor. Although Oxt is implicated in a variety of "non-social" behaviors, such as learning, anxiety, feeding and pain perception, it is Oxt's roles in various social behaviors that have come to the fore recently. Oxt is important for social memory and attachment, sexual and maternal behavior, and aggression. Recent work implicates Oxt in human bonding and trust as well. Human disorders characterized by aberrant social interactions, such as autism and schizophrenia, may also involve Oxt expression. Many, if not most, of Oxt's functions, from social interactions (affiliation, aggression) and sexual behavior to eventual parturition, lactation and maternal behavior, may be viewed as specifically facilitating species propagation.

Effects of an acute stressor on blood pressure and heart rate in rats pretreated with intracerebroventricular oxytocin injections

Oxytocin induces a long-lasting reduction of blood pressure in rats. The aim of the present study was to investigate the effects of an acute stressor on blood pressure and heart rate in rats previously exposed to repeated administration of intracerebroventricular (ICV) oxytocin. For this purpose oxytocin (0.3 microg, ICV) was administered to male rats once a day during 5 days. Blood pressure and heart rate were measured before and after treatment. In addition, blood pressure and heart rate were measured during 30 min after exposure to 10s of noise from an alarm clock. The oxytocin treatment reduced blood pressure significantly (systolic: 108+/-4.6 vs. 121+/-1.8, p<0.01, diastolic: 96+/-5.1 vs. 108+/-3.0, p<0.01), whereas heart rate remained unchanged. In contrast, systolic and diastolic blood pressure increased significantly after the exposure to the ringing alarm clock in the oxytocin-treated rats (p<0.05), and became equal to the blood pressure in controls. In addition, heart rate increased and stayed significantly higher in the oxytocin-treated rats compared to the controls during the 30 min observation period (ANOVA p<0.01). Twenty-four hours later, blood pressure was again significantly lower in the oxytocin-treated rats compared to controls (p<0.01). In conclusion, oxytocin decreased blood pressure without changing pulse rate. However, when the oxytocin-treated rats were subjected to the unexpected noise from a ringing alarm clock blood pressure and heart rate increased significantly. No such effect was observed in the control group. Thus repeated oxytocin treatment can, in spite of decreasing blood pressure during basal conditions, increase cardiovascular reactivity to some types of stressors.

Cocaine treatment alters oxytocin receptor binding but not mRNA production in postpartum rat dams

Gestational cocaine treatment in rat dams results in decreased oxytocin (OT) levels, up-regulated oxytocin receptor (OTR) binding density and decreased receptor affinity in the whole amygdala, all concomitant with a significant increase in maternal aggression on postpartum day six. Rat dams with no gestational drug treatment that received an infusion of an OT antagonist directly into the central nucleus of the amygdala (CeA) exhibited similarly high levels of maternal aggression towards intruders. Additionally, studies indicate that decreased OT release from the hypothalamic division of the paraventricular nucleus (PVN) is coincident with heightened maternal aggression in rats. Thus, it appears that cocaine-induced alterations in OT system dynamics (levels, receptors, production, and/or release) may mediate heightened maternal aggression following cocaine treatment, but the exact mechanisms through which cocaine impacts the OT system have not yet been determined. Based on previous studies, we hypothesized that two likely mechanisms of cocaine's action would be, increased OTR binding specifically in the CeA, and decreased OT mRNA production in the PVN. Autoradiography and in situ hybridization assays were performed on targeted nuclei in brain regions of rat dams on postpartum day six, following gestational treatment twice daily with cocaine (15 mg/kg) or normal saline (1 ml/kg). We now report cocaine-induced reductions in OTR binding density in the ventromedial hypothalamus (VMH) and bed nucleus of the stria terminalis (BNST), but not the CeA. There was no significant change in OT mRNA production in the PVN following cocaine treatment.

Comparisons of dorsal and ventral hippocampus cornu ammonis region 1 pyramidal neuron activity during trace eye-blink conditioning in the rabbit

Previous studies demonstrating a critical role of the hippocampus during trace eye-blink conditioning have focused primarily upon the dorsal portion of the structure. However, evidence suggests that a functional differentiation exists along the septotemporal axis of the hippocampus. In the present study, the activity of 2588 single cornu ammonis region 1 pyramidal neurons of the dorsal hippocampus and ventral hippocampus were recorded during trace and pseudo-eye-blink conditioning of the rabbit. Learning-related increases in dorsal hippocampus neuron firing rates were observed immediately prior to behavioral criterion, and increased over the course of training. Activation of dorsal hippocampus neurons during trace conditioning was also greater than that of ventral hippocampus neurons, including during the trace interval, in well-trained animals. An unexpected difference in the patterns of learning-related activity between hemispheres was also observed. Neurons of the dorsal hippocampus ipsilateral and contralateral to the trained eye, exhibiting significant increases in firing rate [rate increasing neurons], demonstrated the greatest magnitude of activation early and late in training, respectively. Rate increasing neurons of the dorsal hippocampus also exhibited a greater diversity of response profiles, with 69% of dorsal hippocampus rate increasing neurons exhibiting significant increases in firing rate during the conditioned stimulus and/or trace intervals, compared with only 8% of ventral hippocampus rate increasing neurons (the remainder of which were significantly responsive during only the unconditioned stimulus and/or post-unconditioned stimulus intervals). Only modest learning-related activation of ventral hippocampus neurons was observed, reflected as an increase in conditioning stimulus-elicited rate increasing neuron response magnitudes over the course of training. No differences in firing rate between dorsal hippocampus and ventral hippocampus neurons during a 1-day pre-training habituation session were observed. Thus, dorsal hippocampus activation is more robust, suggesting a more substantial role for these neurons in the processing of temporal information during trace eye-blink conditioning.

Longitudinal axis of the hippocampus: both septal and temporal poles of the hippocampus support water maze spatial learning depending on the training protocol

It has been suggested previously that 30% sparing of the hippocampus is enough to support spatial learning of a reference memory task in a water maze provided the spared tissue is located septally (Moser et al. 1995, Proc Natl Acad Sci USA 92:9697-9701). Therefore, the temporal hippocampus may not be involved in spatial memory. Place cells are also found in this part of the structure, and it has been suggested that these place cells have larger, less well-tuned place fields than are found in the septal hippocampus. We tested the possibility that the temporal hippocampus might be involved in spatial learning when the animals are required to distinguish between different contexts. Experiment 1 was a replication of the findings reported by Moser et al., using their protocol (8 trials/day, 6 days) and the groups with 20-40% hippocampus spared septally or temporally (volume assessed by quantitative volumetric techniques). In experiment 2, rats with also 20-40% sparing of the hippocampus either septally or temporally were trained in two water maze concurrently (four trials/day/water maze, 8 days). Rats with 20-40% hippocampus spared temporally were able to learn the two water maze tasks normally, and no difference was observed between rats with septal and temporal hippocampus spared across different measures of performance. In experiment 3, rats with 20-40% hippocampus spared septally or temporally were trained in one water maze as in experiment 1, but using a spaced training protocol similar to that of experiment 2 (four trials/day, 8 days). Rats with temporal hippocampus spared developed a preference for the training quadrant and acquired levels of performance indistinguishable from those of rats with septal hippocampus spared. The results suggest that the temporal hippocampus can support the learning of two, but also one, spatial water maze reference memory task, provided the training protocol is adequate.

Modulation of synaptic transmission by oxytocin and vasopressin in the supraoptic nucleus

It is now generally accepted that magnocellular neurons of the supraoptic and paraventricular nuclei release the neuropeptides oxytocin and vasopressin from their dendrites. Peptide release from their axon terminals in the posterior pituitary and dendrites differ in dynamics suggesting that they may be independently regulated. The dendritic release of peptide within the supraoptic nucleus (SON) is an important part of its physiological function since the local peptides can regulate the electrical activity of magnocellular neurons (MCNs) which possess receptors for these peptides. This direct postsynaptic action would affect the output of peptide in the neurohypophysis. Another way that these peptides can regulate MCN activity would be to modulate afferent inputs unto themselves. Although the influence of afferent inputs (inhibitory and excitatory) on SON magnocellular neuron physiology has been extensively described in the last decade, a role for these locally released peptides on synaptic physiology of this nucleus has been difficult to show until recently, partly because of the difficulty of performing stable synaptic recordings from these cells in suitable preparations that permit extensive examination. We recently showed that under appropriate conditions, oxytocin acts as a retrograde transmitter in the SON. Oxytocin, released from the dendrites of MCNs, decreased evoked excitatory synaptic transmission by inhibiting glutamate release from the presynaptic terminals. It modulated voltage-dependent calcium channels, mainly N-type and to a lesser extent P/Q-type channels, located on glutamatergic terminals. Although evidence is less conclusive, it is possible that vasopressin has similar actions to reduce excitatory transmission. This synaptic depressant effect of oxytocin and/or vasopressin, released from dendrites, would ensure that MCNs regulate afferent input unto themselves using their own firing rate as a gauge. Alternatively, it may only be a subset of afferent terminals that are sensitive to these peptides, thereby providing a means for the MCNs to selectively filter their afferent inputs. Indeed its specificity is partly proven by our observation that oxytocin does not affect spontaneous glutamate release, or GABA release from inhibitory terminals (Brussaard et al., 1996). Thus, the dendrites of MCNs of the supraoptic nucleus serve a dual role as both recipients of afferent input and regulators of the magnitude of afferent input, allowing them to directly participate in the shaping of their output. This adds to a rapidly growing body of evidence in support of the concept of a two-way communication between presynaptic terminals and postsynaptic dendrites, and shows the potential of this nucleus as a model to study such form of synaptic transmission.

Oxytocin and neuroadaptation to cocaine

Oxytocin (OT) has been implicated in neuroadaptive processes such as learning, memory, and social-affiliative behavior as well as in the regulation of physiological responses leading to adaptation to the changing external and internal environment. Drugs of abuse constitute a major challenge to the homeostasis of the body and behavior. Drug tolerance, dependence and addiction may involve neuroadaptive mechanisms related to learning and memory at cellular and systems levels. Considerable effort has been made toward the understanding the neurobiological mechanisms of addictive behavior. Neuropeptides OT and vasopressin (VP) might be involved in these processes based on their effects on neuroadaptation and on their neuroanatomical localization and pharmacological actions. It has been demonstrated that both OT and VP have modulatory effects on opiate and alcohol tolerance and dependence. This chapter summarize the effects of OT, and in lesser extent VP, on neuroadaptation to cocaine, a psychostimulant drug of abuse. We have shown that OT inhibits acute cocaine-induced locomotor hyperactivity, exploratory activity and stereotyped behavior in rodents. Furthermore, OT facilitated, whereas VP inhibited the development of behavioral sensitization to cocaine. In a different model, OT inhibited the development of tolerance to the stereotyped behavior-inducing effects of cocaine as well as cocaine intravenous self-administration in rats. We demonstrated that OT acts through its specific receptors in the basal forebrain and in the hippocampus. OT and VP contents in the hypothalamus and limbic structures were altered by acute and chronic cocaine administration in a dose-dependent and region-selective manner. The differential plasticity of the brain OT-ergic and VP-ergic neurotransmissions in response to cocaine may underlie the differences in the involvement of these neuropeptides in cocaine addiction. Interaction of OT with dopaminergic neurotransmission in the nucleus accumbens, a key brain structure in drug addiction, as well as OT-ergic regulation of hippocampal processes may be among the mechanisms of action through which OT modulates neuroadaptation to cocaine. A better understanding of the role of OT in neuroadaptation to cocaine may provide an insight into both the mechanisms of neuropeptide actions in the brain as well as into the neurobiology of drug addiction.

Historadioautographic localisation of oxytocin and vasopressin binding sites in the central nervous system of the merione (Meriones shawi)

The distribution of vasopressin and oxytocin binding sites in the central nervous system of the merione (Meriones shawi), a rodent adapted to desert life, was studied by means of conventional film radioautography at macroscopic scale and historadioautography at cellular level using radioiodinated ligands highly selective for either oxytocin or type V1 a vasopressin receptors. Both types of binding sites exhibited the same selectivity for endogenous peptides as in the rat. Distribution of oxytocin binding sites was similar in some structures (limbic system, spinal cord) to that described in the rat and in other rodents. Vasopressin binding sites were much more widely distributed in the merione than in the rat brain. In addition to locations common to most rodents (lateral septum and suprachiasmatic nucleus), in merione vasopressin binding sites occurred in several areas known to express oxytocin binding sites in the rat (olfactory system, hypothalamus). Colocalisation of vasopressin and oxytocin binding sites, which occurred in the CA1 and CA2 fields of Ammon's horns of the hippocampus, the caudate-putamen and the fundus striati of the merione, has so far not been reported in any other rodent.

Presence and significance of oxytocin receptors in human neuroblastomas and glial tumors

To determine whether oxytocin (OT) could be added to the list of growth factors acting on neoplastic cells of nervous origin, we investigated the presence of oxytocin receptors (OTR) in human primary neuroblastomas and glioblastomas and related cell lines. OTR were demonstrated both at mRNA level (using a RT-PCR procedure) and at protein level (using immunocytochemical and immunofluorescence procedures). In order to clarify whether OT exerts any biological effect on these tumors through OTR, we also studied cell proliferation in 3 human neuroblastoma cell lines (SK-N-SH, SH-SY5Y, IMR-32) and one human anaplastic astrocytoma cell line (MOG-G-UVW) treated with OT 1 nM to 100 nM for 48 and 96 hr. At these doses, a dose-dependent inhibitory effect on cell proliferation was demonstrated. This inhibition was accompanied by a significant increase in the intracellular concentration of cAMP, which we have reported to be the intracellular mediator of the OT anti-proliferative effect in breast-carcinoma cell lines. Our data indicate that specific OTR are present in human neuroblastomas and glioblastomas. Through these receptors, OT could inhibit cell proliferation and modulate tumor growth.

NGF, cyclic AMP, and phorbol esters regulate oxytocin receptor gene transcription in SK-N-SH and MCF7 cells

Oxytocin receptor (OTR) gene transcription has predominantly been thought to be regulated by estrogen. However, the continuous presence of receptors in certain brain regions after gonadectomy suggests the existence of alternate mechanisms of regulation. We have cloned and sequenced 4 kb of 5'-flanking DNA of the rat OTR gene and identified an internal segment which was absent in the initial publication of this promoter sequence. Sequence analysis of this segment, as well as of a novel upstream region, revealed the presence of a CRE as well as several other potential regulatory elements, including AP-1, AP-2, AP-3, AP-4 sites, an ERE, and a half-SRE (SRE/2). The effects of phorbol 12-myristate 13-acetate (PMA), forskolin, and NGF treatment on this promoter were tested in transfection experiments in MCF7 and SK-N-SH cells. Transcription of the full-length OTR promoter was induced by forskolin and by the phorbol ester PMA, and a synergistic (17-fold) effect was observed in MCF7 cells treated with both agents. Receptor binding studies using the OTR antagonist 125I-labeled ornithine vasotocin, and Western blot analyses of OTRs in MCF7 cells, showed that PMA and forskolin also increased the density of endogenous human oxytocin receptors. Mutational analyses of the CRE and half-SRE sites in this promoter indicated that these elements function as enhancers and support forskolin and NGF effects, respectively, on transcription. These studies have identified a novel region of the rat OTR promoter containing elements which impart cAMP and/or phorbol ester inducibility of OTR gene transcription. A potential role of the PKA and/or PKC pathways in OTR gene regulation is suggested.

Dendritically released peptides act as retrograde modulators of afferent excitation in the supraoptic nucleus in vitro

Oxytocin (OXT) and vasopressin (VP) are known to be released from dendrites of magnocellular neurons. Here, we show that these peptides reduced evoked EPSCs by a presynaptic mechanism, an effect blocked by peptide antagonists and mimicked by inhibition of endogenous peptidases. Dendritic release of peptides, elicited with depolarization achieved by high frequency stimulation of afferents or with current injection into an individual neuron, induced short-term synaptic depression similar to that seen following exogenous peptide application and was prevented by peptide antagonists. Thus, dendritically released peptides depress evoked EPSCs in magnocellular neurons by activating presynaptic OXT and/or VP receptors. Such a retrograde modulatory action on afferent excitation may serve as a feedback mechanism to permit peptidergic neurosecretory neurons to autoregulate their own activity.

Polymorphism and genetic mapping of the human oxytocin receptor gene on chromosome 3

Centrally administered oxytocin has been reported to facilitate affiliative and social behaviours, in functional harmony with its well-known peripheral effects on uterine contraction and milk ejection. The biological effects of oxytocin could be perturbed by mutations occurring in the sequence of the oxytocin receptor gene, and it would be of interest to establish the position of this gene on the human linkage map. Therefore we identified a polymorphism at the human oxytocin receptor gene. A portion of the 3' untranslated region containing a 30 bp CA repeat was amplified by polymerase chain reaction (PCR), revealing a polymorphism with two alleles occurring with frequencies of 0.77 and 0.23 in a sample of Caucasian CEPH parents (n = 70). The CA repeat polymorphism we detected was used to map the the human oxytocin receptor to chromosome 3p25-3p26, in a region which contains several important genes, including loci for Von Hippel-Lindau disease (VHL) and renal cell carcinoma.

The ependyma: a protective barrier between brain and cerebrospinal fluid

This review summarizes the current scientific literature concerning the ependymal lining of the cerebral ventricles of the brain with an emphasis on selective barrier function and protective roles for the common ependymal cell. Topics covered include the development, morphology, protein and enzyme expression including reactive changes, and pathology. Some cells lining the neural tube are committed at an early stage to becoming ependymal cells. They serve a secretory function and perhaps act as a cellular/axonal guidance system, particularly during fetal development. In the mature mammalian brain ependymal cells possess the structural and enzymatic characteristics necessary for scavenging and detoxifying a wide variety of substances in the CSF, thus forming a metabolic barrier at the brain-CSF interface.

Oxytocin and vasopressin binding sites in the hypothalamus of the rat: histoautoradiographic detection

Localization of oxytocin and vasopressin binding sites has so far been studied in the rat brain by means of film autoradiographs. The availability of selective iodinated ligands with high specific activity allowed us to develop the histoautoradiographical technique and to reinvestigate at the microscopic scale the distribution of these sites in the hypothalamus. Most oxytocin binding sites were localized in delimited nuclei, e.g., the medial preoptic, the ventromedial, the ventral premammillary, the supramammillary, and the medial mammillary nuclei. In addition, a weak diffuse specific labeling occurred in the medial preoptic and the anterior hypothalamic areas. The vasopressin binding sites (of the V1a type) were detected in delimited nuclei, e.g., the suprachiasmatic, the stigmoid, and the arcuate nuclei, but they were also diffusely distributed in the lateral hypothalamic and the dorsochiasmatic areas. The locations of neurohypophysial peptides binding sites detected by light microscopy are compared with those previously obtained by film autoradiography.

Oxytocin receptors on oxytocin neurones: histoautoradiographic detection in the lactating rat

1. The purpose of the present study was the detection at the cellular scale of the oxytocin (OT) receptors involved in the facilitatory effect of this neuropeptide on its own release during the milk ejection reflex. 2. OT binding sites were demonstrated in brain sections by using a highly selective 125I-labelled OT antagonist detected by film- and histoautoradiography. 3. Film autoradiographs revealed the presence of OT binding sites in the hypothalamic magnocellular (supraoptic, paraventricular and anterior commissural) nuclei in lactating rats, suckled or not. This detection was only possible after acute i.c.v. injection of OT antagonist which probably induced an upregulation of the OT binding sites to autoradiographically detectable levels. 4. Combined application of histoautoradiographic and immunohistochemical techniques showed that the OT binding sites were concentrated on OT magnocellular neurones. Labelling concerned cell bodies and dendrites but not the axons and endings in the pituitary neural lobe. 5. The presently detected somatodendritic autoreceptors on OT neurones probably mediate the facilitatory effect of OT on its own release during the milk ejection reflex.

Neuron-glia interactions in the release of oxytocin and vasopressin from the rat neural lobe: the role of opioids, other neuropeptides and their receptors

The release of the neurohormones oxytocin and vasopressin from the neural lobe into the circulation is regulated in a complex manner, which has only been partly elucidated. At the level of the neural lobe, regulation of release can occur by various endogenous compounds that act on specific receptors present on the nerve terminals themselves. In addition, release may be modulated by an alternative pathway in which the local glia cells, the pituicytes, are involved. It is especially the latter pathway that is discussed in detail in this commentary.

Neuropsychology of maternal behavior in the rat: c-fos expression during mother-litter interactions

This series of studies used the pattern of nuclear Fos-like immunoreactivity (Fos-lir) to map the functional pathways in the brain that mediate the onset and retention of maternal behavior. In the first two experiments, parturient rat dams were exposed to either pups or to other stimuli on Day 1 postpartum. Dams interacting with pups were either intact or sustained ventral somatosensory, olfactory, or combined desensitizations. Results showed that 1) all intact pup-interacting dams showed elevated levels of Fos-lir in the medial preoptic area (MPOA) and the medial and cortical amygdala as compared to control groups, and 2) olfactory and ventral somatosensory desensitization, either alone or in combination, did not decrease Fos-lir in the MPOA. However, olfactory desensitizations did decrease Fos-lir in the medial amygdala and the combined desensitizations significantly reduced Fos-lir in both the basolateral and central amygdala. In the third study, dams were either exposed to pups or to other stimuli and were subsequently reexposed to pups or to pup cues. Regardless of prior maternal experience, females who were able to interact with pups upon reexposure showed increased Fos-lir in the MPOA, the basolateral and central nuclei of the amygdala, and the nucleus accumbens when compared to females which did not interact with pups. Taken together, these studies suggest that the neuroanatomy of maternal behavior is a complex one, involving multiple systems that interconnect with the MPOA and that mediate the many behavioral processes activated when an animal responds maternally.

Localization of oxytocin binding sites in the thoracic and upper lumbar spinal cord of the adult and postnatal rat: a histoautoradiographic study

Oxytocin binding sites were detected by autoradiography on films and emulsion-coated sections in the spinal cord of adult and postnatal rats from C8 to L2, using a highly selective 125I-labelled oxytocin antagonist. Oxytocin binding sites were detected on all transverse sections in the dorsal horn, where labelling was scattered over laminae I and II. The autonomic areas, i.e. the intermediolateral cell column, the central grey (lamina X) and the nucleus intercalatus were labelled. Binding in the intermediolateral cell column was most frequently observed on sections from T9 to T11 in adult and T7 to T8 in postnatal rats. In this location, oxytocin binding sites were highly concentrated on cell bodies of putative sympathetic preganglionic neurons; however, not all of these cells were labelled. Diffuse labelling occurred on the dorsal part of the central grey, mainly between T8 and L2. Isolated labelled cells belonging to the nucleus intercalatus were scattered between the central canal and the intermediolateral cell column. In addition, oxytocin binding sites were found on some motoneurons of the lateral group of T12-T13, but only in postnatal rats. The distribution of oxytocin binding sites in the rat spinal cord coincides with that of the oxytocin innervation and strongly suggests a modulatory role of this peptide in sensory and autonomic functions.

Role of oxytocin in the neuroadaptation to drugs of abuse

Oxytocin (OXT), a neurohypophyseal hormone, has a wide range of behavioral effects outside its classic peripheral endocrine functions. OXT involvement in adaptive central nervous system processes has been demonstrated as an inhibitory, amnestic action on learning and memory in different paradigms. Because adaptation and learning are likely to be involved in the neural events leading to drug tolerance and dependence, the question logically arose whether OXT is able to influence the development of tolerance of and dependence on abused drugs. In this review, we summarize our results on the effects of OXT on opiate (including morphine, heroin, and the endogenous opiates beta-endorphin and enkephalin) tolerance and dependence, heroin self-administration, psychostimulant-induced behavioral changes, and behavioral tolerance and sensitization. The sites and mechanisms of action and the possible physiological role of OXT are also discussed. In the first part of this review the effects of exogenously administered OXT on both the acute and chronic behavioral effects of opiates and psychostimulants have been summarized. OXT inhibited the development of tolerance to morphine, heroin, beta-endorphin, and enkephalin, OXT also inhibited the development of cross-tolerance between the predominantly mu-agonist heroin and the predominantly delta-agonist enkephalin in mice. Naloxone-precipitated morphine withdrawal syndrome was also attenuated by OXT. Heroin self-administration was decreased by OXT administration in heroin-tolerant rats. OXT inhibited cocaine-induced exploratory activity, locomotor hyperactivity, and stereotyped behavior in rats and in mice. Behavioral tolerance to cocaine was also attenuated by OXT. On the contrary, OXT stimulated the development of behavioral sensitization to cocaine. OXT did not alter the stereotyped behavior induced by amphetamine. In the second series of experiments, the sites of action of OXT on drug-related behavior were investigated. Intracerebro-ventricular (ICV) and intracerebral (IC) administration of an OXT-receptor antagonist inhibited the effects of peripherally administered OXT on morphine tolerance, heroin self-administration, and cocaine-induced sniffing behavior. This suggests the central, intracerebral location of OXT target sites. Local IC microinjection of OXT in physiological doses into the posterior olfactory nucleus, tuberculum olfactorium, nucleus accumbens, central amygdaloid nucleus, and the hippocampus inhibited the development of tolerance to and dependence on morphine as well as cocaine-induced sniffing behavior and tolerance to cocaine. The physiological role of endogenous OXT in acute morphine tolerance has also been demonstrated, since OXT antiserum (ICV) and OXT-receptor antagonist (injected into the basal forebrain structures) potentiated the development of morphine tolerance. Finally, we investigated the possible mechanisms of action of OXT on drug related behavior. Both morphine tolerance and dependence, and cocaine administration, increased dopamine utilization in the mesencephalon and in the nucleus accumbens, respectively. OXT treatment decreased the alpha-methylparatyrosine-induced dopamine utilization in the mesencephalon and in the nucleus accumbens-septal complex. Chronic OXT treatment decreased the number of apparent binding sites of dopamine in the basal forebrain area. It also inhibited a cocaine-induced increase in dopamine utilization in the nucleus accumbens, but not in the striatum. In light of this information, it appears that OXT inhibits the development of opiate tolerance, dependence, and self-administration as well as the acute behavioral actions of and chronic tolerance to cocaine. This suggests the possible role of this neuropeptide in the regulation of drug abuse. Therefore, OXT may act as a neuromodulator on dopaminergic neurotransmission in limbic-basal forebrain structures to regulate adaptive CNS processes leading to drug addiction.

The release of oxytocin from spinal cord synaptosomes by high KCl depolarizing stimulus: a calcium dependent process

Oxytocin (OT) release from synaptosomes isolated from the thoracic (T) and lumbosacral (LS) regions of the spinal cord was evoked by 56 mM potassium chloride (KCl). The release mechanism was shown to be a calcium dependent process. The ability of high KCl to evoke OT release from isolated nerve terminals in a calcium dependent manner provides additional support for the role of OT as a neurotransmitter in the spinal cord.

Histoautoradiographic detection of oxytocin- and vasopressin-binding sites in the telencephalon of the rat

Localization of oxytocin- and vasopressin-binding sites has so far been studied in the rat brain by means of film autoradiographs. The disposal of iodinated ligands with high specificity has allowed us to develop histoautoradiography on emulsion-coated sections and to reinvestigate on a microscopic scale the distribution of these sites in the telencephalon (septum, striatopallidal system, amygdala and hippocampus). This technique showed that oxytocin and vasopressin labelling presented distinct distributions and coincided with delimited zones, corresponding to anatomical subdivisions defined on cytoarchitectural and immunocytochemical bases. Vasopressin sites were seen in the dorsal and intermediate parts of the lateral septum and the juxtacapsular nucleus of the bed nucleus of the stria terminalis. Oxytocin sites were located in the ventral and intermediate parts of the lateral septum, the oval and the principal nuclei of the bed nucleus of the stria terminalis and the septofimbrial nucleus. In the striatopallidal system, vasopressin sites were found in the accumbens nucleus and the fundus striati, whereas oxytocin sites were in the accumbens nucleus, the head, and the posterolateral parts of the caudate-putamen, the striatal cell bridges, and the olfactory tubercle. In the amygdala, vasopressin sites were not found, but oxytocin sites were located in the central, medial, and basomedial nuclei. In the hippocampus, vasopressin sites were located in the dentate gyrus (polymorph and molecular layers), and oxytocin sites, in the subiculum (molecular and pyramidal layers) and in the field CA1 of Ammon's horn (lacunosum moleculare and pyramidal layers). The localization of the binding sites at the microscopic level permitted us to reinvestigate whether or not correlation existed in a same area between innervation, electrophysiological effects, and presence of binding sites.

Immunoelectron microscopic demonstration of oxytocin and vasopressin in pituicytes and in nerve terminals forming synaptoid contacts with pituicytes in the rat neural lobe

A pre-embedding immunoelectron microscopic technique was used to obtain morphological evidence for a role of oxytocin and vasopressin in the regulation of their own or each others release from the neural lobe. No synaptoid contacts of oxytocin- or vasopressin-containing axons with other neuronal structures were observed. However, synaptoid contacts of oxytocin- and vasopressin-containing nerve terminals and Herring bodies with pituicytes were frequently observed. These findings suggest that the pituicyte may participate in auto- and/or cross-regulation of oxytocin and vasopressin release. Moreover, oxytocin and vasopressin precursor-derived peptides were found in the cytoplasm of some pituicytes, an unexpected finding that will be discussed.

Electrical activity of neurons in the ventrolateral septum and bed nuclei of the stria terminalis in suckled rats: statistical analysis gives evidence for sensitivity to oxytocin and for relation to the milk-ejection reflex

Our previous results obtained by lesioning or stimulating the ventrolateral part of the lateral septum and the bed nuclei of the stria terminalis suggested that this area is involved in the control of milk ejection pattern in rats. The present study was undertaken with the aim of testing ventrolateral part of the lateral septum-bed nuclei of the stria terminalis neurons as a putative link of the neuronal network controlling the bursting activity of oxytocin neurons in suckled lactating rats (anaesthetized with urethane). Ventrolateral part of the lateral septum-bed nuclei of the stria terminalis neurons were recorded simultaneously with hypothalamic oxytocin neurons in either the paraventricular or supraoptic nucleus in rats with (n = 26) or without (n = 29) periodic milk ejections. Analysis of their firing pattern enabled differentiation of two subgroups: type I, characterized by numerous high frequency spikes, often grouped in clusters; and type II with very few or no high frequency clusters of spikes. The probability density function of the interspike intervals of both patterns could be modelled using a mixture of two log-normal distributions, the parameters of which differed significantly. The presence of absence of milk ejections did not influence the overall mean level of activity (2.0 +/- 0.5 and 1.9 +/- 0.4 spikes/s, respectively). However, the characteristics of the type I firing pattern were affected by the presence of the milk-ejection reflex. The average level of activity was not always constant and 16/55 ventrolateral part of the lateral septum-bed nuclei of the stria terminalis neurons displayed cyclical activity (from 0.6 +/- 0.2 to 4.0 +/- 0.5 spikes/s) both in the presence (n = 8) and absence (n = 8) of the milk-ejection reflex. In five of eight neurons recorded during milk-ejection reflex, the cycles in firing were clearly correlated with the bursting of oxytocin neurons. These five neurons exhibited the type I firing pattern. The three remaining neurons and the eight neurons recorded in the absence of milk-ejection reflex displayed the type II firing pattern. Oxytocin (1-2 ng = 0.45-0.9 mU) was injected into the third ventricle (i.c.v.) in order to examine the possible involvement of ventrolateral part of the lateral septum-bed nuclei of the stria terminalis neurons in the facilitatory effect of oxytocin on the reflex.(ABSTRACT TRUNCATED AT 400 WORDS)

Oxytocin--a neuropeptide for affiliation: evidence from behavioral, receptor autoradiographic, and comparative studies

Oxytocin (OT) is a nine amino acid peptide synthesized in hypothalamic cells which project either to the neurohypophysis or to sites within the central nervous system. Although neurohypophyseal OT release has long been associated with uterine contraction and milk ejection, the function of intracerebral OT remains unclear. On the basis of behavioral, cellular, and comparative studies, this review suggests that brain OT influences the formation of social bonds. The first part of this review examines evidence linking central OT to several forms of affiliation. Central administration of OT induces maternal and reproductive behaviors in rats primed with gonadal steroids. OT antagonists and hypothalamic lesions block the initiation of maternal and reproductive behaviors but have no effects on these behaviors once established. Our new studies in rat pups demonstrate that central OT selectively decreases the separation response, an effect which mimics social contact. These studies of parental, reproductive, and attachment behaviors suggest that exogenous OT has "prosocial" effects and that endogenous OT may be essential for initiating social interaction. In a second series of experiments, we investigated the cellular mechanisms for OT's effects on social behavior by means of autoradiographic receptor binding. In the rat forebrain, OT receptors are expressed in several limbic regions believed to be involved in the integration of sensory processing. The regulation of these receptors is surprisingly resistant to either ablation of OT cells or repeated central administration of OT. However, receptors in two regions, the bed nucleus of the stria terminalis (BNST) and the ventromedial nucleus of the hypothalamus (VMN), appear selectively induced by exogenous or endogenous increases in gonadal steroids. At parturition, binding to OT receptors increases 84% in the BNST, and at estrus, binding increases 35% in the VMN. These results demonstrate that physiologic changes in gonadal steroids can alter receptor expression in anatomically discrete target fields and thereby direct responsiveness to endogenous neuropeptide release. A model for OT's effects on social behavior is proposed, which relies on the heterologous regulation of the brain OT receptor. A third series of experiments tested the hypothesis that brain OT influences affiliation by comparing prairie and montane voles, two closely related species with dichotomous systems of social organization. Although no differences appear in the presynaptic expression of the neuropeptide, OT receptors are distributed in complementary patterns in the two species. In the highly affiliative prairie vole, receptors are most evident in the BNST and one of its primary afferents, the lateral amygdala, highlighting a circuit previously implicated in maternal behavior.(ABSTRACT TRUNCATED AT 400 WORDS)

Autoradiographic detection of oxytocin- and vasopressin-binding sites in various subnuclei of the bed nucleus of the stria terminalis in the rat. Effects of functional and experimental sexual steroid variations

Abstract Oxytocin- and vasopressin-binding sites were detected by autoradiography on films and on emulsion-coated sections of rat brains using highly selective [(125)|]-labelled oxytocin and vasopressin antagonists. Two distinct areas with high concentrations of oxytocin-binding sites were detected in the bed nucleus of the stria terminalis: 1) the principal encapsulated nucleus and the associated cell-sparse zone in the posterior medial part, and 2) the oval nucleus in the anterior lateral part. A weak diffuse labelling was, in addition, detected around the oval nucleus in the anterior lateral and anterior dorsal areas. The vasopressin-binding sites were restricted to the anterior lateral part of the bed nucleus of the stria terminalis where they were highly concentrated in the juxtacapsular nucleus and present with lower density in a discrete cell group dorsal to the oval nucleus. Autoradiographic analyses of the bed nucleus of the stria terminalis from pregnant, lactating and ovariectomized rats (oestradiol treated or not) indicated that only the oxytocin-binding sites in the principal encapsulated nucleus and the associated cell-sparse zone were oestrogen-dependent. These observations are in agreement with earlier data suggesting that the two major divisions of the bed nucleus of the stria terminalis are involved in distinct regulations. The anterior lateral part, including the oval nucleus in which oxytocin receptors are not oestrogen-dependent, is, rather, involved in central autonomie regulations. The posterior medial part, where oestrogen-dependent oxytocin receptors are concentrated in the principal encapsulated nucleus and the associated cell-sparse zone, is implicated in neuroendocrine regulations and in reproductive behaviour.

Oxytocin excites neurons located in the ventromedial nucleus of the Guinea-pig hypothalamus

Abstract The area of the ventromedial nucleus of the hypothalamus in the guinea-pig was shown in autoradiographs to contain high affinity binding sites for oxytocin. In order to ascertain whether these sites may represent neuronal receptors, single-cell extracellular recordings were obtained from ventromedial neurons in coronal slices of the hypothalamus of adult guinea-pigs. Oxytocin applied in the nanomolar range excited about half of the neurons tested; none were inhibited. The response to the peptide was reversible and concentration-dependent. It was exerted directly since it persisted under the condition of synaptic isolation. Moreover, the effect was specific since it could be mimicked by a selective oxytocin agonist and since vasopressin was usually at least 10-fold weaker than oxytocin. These findings suggest that the binding sites for oxytocin detected by light microscopic autoradiography in the guinea-pig hypothalamic ventromedial nucleus represent functional receptors.

Neurophysins, rather than Receptors, are Involved in [H]Oxytocin and [H]Vasopressin Binding Detected by Autoradiography in the Hypothalamo-Neurohypophyseal System

Abstract The goal of the present experiments was to analyse the binding of oxytocin (OT) and vasopressin (VP) in the hypothalamo-neurohypophyseal system to determine whether [(3)H]OT and [(3)H]VP binding in this system involved interaction with receptor sites or with neurophysins. Using quantitative autoradiography, several experiments were performed to compare [(3)H]OT- and [(3)H]VP-binding characteristics in this system and in brain areas containing identified receptor sites. Saturation experiments indicated much lower affinity of [(3)H]OT and [(3)H]VP binding in the magnocellular nuclei and neural lobe than on brain receptors. Competition experiments using selective ligands indicated interaction with neurophysins rather than with receptors in the hypothalamo-neurohypophyseal system. This system was never labelled in the presence of a [(125)I]OT antagonist, a selective OT receptor ligand. In contrast with receptors elsewhere in the brain, the magnocellular nuclei were labelled by [(3)H]OT and [(3)H]VP in the absence of MgCI(2). In the pituitary neural lobe, density of binding sites was moreover obviously related to the amount of neurosecretory granules, as seen in acutely dehydrated rats. Taken together, these data strongly suggest that in the hypothalamo-neurohypophyseal system [(3)H]OT and [(3)H]VP bind to neurophysins rather than to specific receptors.